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Title: Uncertainty Budget and Efficiency Analysis for the 239Pu (n,2ny) Partial Reaction Cross-Section Measurements

Abstract

The {sup 239}Pu(n,2n{gamma}){sup 238}Pu partial reaction cross-section, {sigma}{sub (n,2n{gamma})}, has been measured as a function of neutron energy for several transitions in {sup 238}Pu. Partial {gamma}-ray cross sections for yrast, ''collector'' transitions, can provide especially valuable constraints on the magnitude and shape of the total (n,2n) reaction cross-section. In essence, nuclear reaction models will be used to infer the shape and magnitude of the total (n,2n) reaction cross-section from the measured partial {gamma}-ray cross-sections. The reason for undertaking this somewhat indirect approach is that previous measurements of the {sup 239}Pu(n,2n{gamma}) have been hampered by a variety of constraints. Activation measurements have several hurdles: (1) intense flux and long counting times are required to overcome the relatively long half-life of {sup 238}Pu (87 years) and (2) isotopically pure samples of {sup 239}Pu in an environment free of {sup 238}Pu contamination are difficult to come by. Neutron counting experiments are subject to significant uncertainties because (1) large background statistics from fission neutrons and (2) the experimental fission neutron multiplicity spectrum is subject to systematic errors because the flux of low-energy neutrons which induce fissions in thermally-fissile {sup 239}Pu is very difficult to characterize. In this measurement, spallation neutrons are provided by themore » LANSCE/WNR facility, and reaction neutron energies are determined via time-of-flight. Neutron flux is monitored in-beam with one {sup 235}U fission chamber and one {sup 238}U fission chamber. The {sup 238}U is not sensitive to background from low-energy neutrons, whereas the {sup 235}U fission chamber has better statistics. Hence, in essence the partial {gamma}-ray cross sections are normalized to the evaluated fission cross sections of {sup 235}U and {sup 238}U. As a check of our normalization to provide additional constraints to the nuclear reaction modeling, benchmark measurements of {sup nat}Fe(n, n{prime}{gamma}) and {sup 235}U(n,2n{gamma}) have also been undertaken. The secondary {gamma}-rays are measured with the GEANIE array. GEANIE consists of eleven Compton-suppressed planar detectors, nine suppressed and six unsuppressed co-axial detectors. Any absolute cross section measurement requires a complete understanding of array performance, flux normalization, and target effects. Important items to consider in this experiment include intrinsic detector efficiency, beam and detector geometry corrections, target attenuation, and deadtime. Radioactive targets give rise to significant counting rates in the GEANIE array resulting a large deadtime. The magnitude, energy dependence, and uncertainties of these effects and other corrections are the subject of this paper.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab., CA (US)
Sponsoring Org.:
USDOE Office of Defense Programs (DP) (US)
OSTI Identifier:
793116
Report Number(s):
UCRL-ID-139906
TRN: US0204457
DOE Contract Number:
W-7405-Eng-48
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: 1 May 2000
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; 73 NUCLEAR PHYSICS AND RADIATION PHYSICS; COUNTING RATES; CROSS SECTIONS; EFFICIENCY; ENERGY DEPENDENCE; FISSION CHAMBERS; FISSION NEUTRONS; NEUTRON FLUX; NUCLEAR REACTIONS

Citation Formats

McNabb, D.P., Archer, D.E., Becker, J.A., Bernstein, L.A., and Garrett, P.E.. Uncertainty Budget and Efficiency Analysis for the 239Pu (n,2ny) Partial Reaction Cross-Section Measurements. United States: N. p., 2000. Web. doi:10.2172/793116.
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McNabb, D.P., Archer, D.E., Becker, J.A., Bernstein, L.A., & Garrett, P.E.. Uncertainty Budget and Efficiency Analysis for the 239Pu (n,2ny) Partial Reaction Cross-Section Measurements. United States. doi:10.2172/793116.
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McNabb, D.P., Archer, D.E., Becker, J.A., Bernstein, L.A., and Garrett, P.E.. Mon . "Uncertainty Budget and Efficiency Analysis for the 239Pu (n,2ny) Partial Reaction Cross-Section Measurements". United States. doi:10.2172/793116. https://www.osti.gov/servlets/purl/793116.
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@article{osti_793116,
title = {Uncertainty Budget and Efficiency Analysis for the 239Pu (n,2ny) Partial Reaction Cross-Section Measurements},
author = {McNabb, D.P. and Archer, D.E. and Becker, J.A. and Bernstein, L.A. and Garrett, P.E.},
abstractNote = {The {sup 239}Pu(n,2n{gamma}){sup 238}Pu partial reaction cross-section, {sigma}{sub (n,2n{gamma})}, has been measured as a function of neutron energy for several transitions in {sup 238}Pu. Partial {gamma}-ray cross sections for yrast, ''collector'' transitions, can provide especially valuable constraints on the magnitude and shape of the total (n,2n) reaction cross-section. In essence, nuclear reaction models will be used to infer the shape and magnitude of the total (n,2n) reaction cross-section from the measured partial {gamma}-ray cross-sections. The reason for undertaking this somewhat indirect approach is that previous measurements of the {sup 239}Pu(n,2n{gamma}) have been hampered by a variety of constraints. Activation measurements have several hurdles: (1) intense flux and long counting times are required to overcome the relatively long half-life of {sup 238}Pu (87 years) and (2) isotopically pure samples of {sup 239}Pu in an environment free of {sup 238}Pu contamination are difficult to come by. Neutron counting experiments are subject to significant uncertainties because (1) large background statistics from fission neutrons and (2) the experimental fission neutron multiplicity spectrum is subject to systematic errors because the flux of low-energy neutrons which induce fissions in thermally-fissile {sup 239}Pu is very difficult to characterize. In this measurement, spallation neutrons are provided by the LANSCE/WNR facility, and reaction neutron energies are determined via time-of-flight. Neutron flux is monitored in-beam with one {sup 235}U fission chamber and one {sup 238}U fission chamber. The {sup 238}U is not sensitive to background from low-energy neutrons, whereas the {sup 235}U fission chamber has better statistics. Hence, in essence the partial {gamma}-ray cross sections are normalized to the evaluated fission cross sections of {sup 235}U and {sup 238}U. As a check of our normalization to provide additional constraints to the nuclear reaction modeling, benchmark measurements of {sup nat}Fe(n, n{prime}{gamma}) and {sup 235}U(n,2n{gamma}) have also been undertaken. The secondary {gamma}-rays are measured with the GEANIE array. GEANIE consists of eleven Compton-suppressed planar detectors, nine suppressed and six unsuppressed co-axial detectors. Any absolute cross section measurement requires a complete understanding of array performance, flux normalization, and target effects. Important items to consider in this experiment include intrinsic detector efficiency, beam and detector geometry corrections, target attenuation, and deadtime. Radioactive targets give rise to significant counting rates in the GEANIE array resulting a large deadtime. The magnitude, energy dependence, and uncertainties of these effects and other corrections are the subject of this paper.},
doi = {10.2172/793116},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon May 01 00:00:00 EDT 2000},
month = {Mon May 01 00:00:00 EDT 2000}
}
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DOI:  10.2172/793116
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  • ; ; ; ...
    Absolute partial {gamma}-ray cross sections for production of discrete {gamma} rays in the {sup 239}Pu(n,2n{gamma}i){sup 238}Pu reaction have been measured. The experiments were performed at LANSCE/WNR on the 60R flight line. Reaction {gamma}-rays were measured using the large-scale Compton-suppressed array of Ge detectors, GEANIE. The motivation for this experiment, an overview of the partial {gamma}-ray cross-section measurement, and an introduction to the main experimental issues will be presented. The energy resolution of the Ge detectors allowed identification of reaction {gamma} rays above the background of sample radioactivity and fission {gamma} rays. The use of planar Ge detectors with their reducedmore » sensitivity to neutron interactions and improved line shape was also important to the success of this experiment. Absolute partial {gamma}-ray cross sections are presented for the 6{sub 1}{sup +} {yields} 4{sub 1}{sup +} member of the ground state rotational band in {sup 238}Pu, together with miscellaneous other {gamma}-ray partial cross sections. The n,2n reaction cross section shape and magnitude as a function of neutron energy was extracted from these partial cross sections using nuclear modeling (enhanced Hauser-Feshbach) to relate partial {gamma}-ray cross sections to the n,2n cross section. The critical nuclear modeling issue is the ratio of a partial cross section to the reaction channel cross section, and not the prediction of the absolute magnitude.« less

  • ; ; ; ...
    The goal of this project was to develop a new approach to measuring (n,2n) reactions for isotopes of interest. We set out to measure the 239Pu(n,2n) and 241Pu(n,2n) cross sections by directly detecting the 2n neutrons that are emitted. With the goal of improving the 239Pu(n,2n) cross section and to measure the 241Pu(n,2n) cross section for the first time. To that end, we have constructed a new neutron-charged-particle detector array called NeutronSTARS. It has been described extensively in Casperson et al. [1] and in Akindele et al. [2]. We have used this new neutron-charged-particle array to measure the 241Pu andmore » 239Pu fission neutron multiplicity as a function of equivalent incident-neutron energy from 100 keV to 20 MeV. We have made a preliminary determination of the 239Pu(n,2n) and 241Pu(n,2n) cross sections from the surrogate 240Pu(α,α’2n) and 242Pu(α,α’2n) reactions respectively. The experimental approach, detector array, data analysis, and results to date are summarized in the following sections.« less

  • ; ;
    This report presents the latest {sup 239}Pu(n,2n){sup 238}Pu cross sections inferred from calculations performed with the nuclear reaction-modeling code system, IDA, coupled with experimental measurements of partial {gamma}-ray cross sections for incident neutron energies ranging from 5.68 to 17.18 MeV. It is found that the inferred {sup 239}Pu(n,2n){sup 238}Pu cross section peaks at E{sub inc} {approx} 11.4 MeV with a peak value of approximately 326 mb. At E{sub inc} {approx} 14 MeV, the inferred {sup 239}Pu(n,2n){sup 238}Pu cross section is found to be in good agreement with previous radio-chemical measurements by Lockheed. However, the shape of the inferred {sup 239}Pu(n,2n){supmore » 238}Pu cross section differs significantly from previous evaluations of ENDL, ENDF/B-V and ENDF/B-VI. In our calculations, direct, preequilibrium, and compound reactions are included. Also considered in the modeling are fission and {gamma}-cascade processes in addition to particle emission. The main components of physics adopted and the parameters used in our calculations are discussed. Good agreement of the inferred {sup 239}Pu(n,2n){sup 238}Pu cross sections derived separately from IDA and GNASH calculations is shown. The two inferences provide an estimate of variations in the deduced {sup 239}Pu(n,2n){sup 238}Pu cross section originating from modeling.« less

  • A procedure is presented to deduce the reaction-channel cross section from measured partial {gamma}-ray cross sections. In its simplest form, the procedure consists in adding complementary measured and calculated contributions to produce the channel cross section. A matrix formalism is introduced to provide a rigorous framework for this approach. The formalism is illustrated using a fictitious product nucleus with a simple level scheme, and a general algorithm is presented to process any level scheme. In order to circumvent the cumbersome algebra that can arise in the matrix formalism, a more intuitive graphical procedure is introduced to obtain the same reactionmore » cross-section estimate. The features and limitations of the method are discussed, and the technique is applied to extract the {sup 235}U (n,2n) and {sup 239}Pu(n,2n) cross sections from experimental partial {gamma}-ray cross sections, coupled with (enhanced) Hauser-Feshbach calculations.« less

  • ; ; ; ...
    Previous experimental efforts to measure the {sup 239}Pu(n,2n) reaction cross section have relied on the detection of evaporated neutrons [Mat72,Fre85]. These efforts were hampered by the presence of the large ({sigma} {approx} 2 barns) neutron-induced fission channel which produces on average 3-4 neutrons [How71]. This paper is one of three manuscripts that document an effort to determine the {sup 239}Pu(n,2n) channel cross section using the GErmanium Array for Neutron Induced Excitations (GEANIE) spectrometer [Bec97] at the Los Alamos Neutron Science Center/Weapons Neutron Research (LANSCE/WNR) [Lis90] facility. In this document we report the measurement of several {sup 239}Pu(n,xn{gamma}){sup 240-x}Pu partial crossmore » sections. The other two papers report results for a parallel proof-of-principle experiment using a {sup 235}U target as a surrogate for {sup 239}Pu [Youn00] and a detailed calculation and measurement of the efficiency of the GEANIE spectrometer [McN00]. Results from these two works are frequently referred to in this paper. A later report from Becker et al. will use the partial (n2n{gamma}) cross sections reported here, together with the predictions of the GNASH [Cha99,Cha00] and IDA [Ros99] reaction models, to extract a total cross section for the (n,2n) channel. The results of three experiments are reported here. Two were carried out in 1998 using a 0.010 inch (referred to as ''thin'') and 0.020 inch (referred to as ''thick'') {sup 239}Pu target. The remaining one was run in 1999 using the same thin target. This work is being carried out in parallel with a similar effort to measure (n,xn{gamma}) cross sections on a {sup 235}U target being analyzed by Younes et al., [You00]. This experiment was performed using the same {gamma}-ray spectrometer and a similar analysis approach, thereby allowing a detailed comparison between the two data sets and a check on the techniques used. The report is comprised of three portions. In the first part the experimental techniques used for all three runs will be presented. The next section will describe how the partial (n,xn{gamma}) cross sections were extracted from a combination of all three data sets, including a detailed discussion of the level spectroscopy of levels in {sup 238}Pu populated in this experiment. The last portion of the report will focus on a comparison between the observed partial cross sections and the predictions of the GNASH reaction model [Cha99,Cha00]. Several methods will be suggested in which the model predictions may be coupled with the measured partial cross sections to extract a (n,2n) channel cross section. A separate appendix will present the results from the three separate experiments individually.« less